Aligned high-density ZnO nanorods were successfully synthesized on porous aluminum oxide (PAO) template. The growth process involves carbonthermal reduction of ZnO as a Zn vapor source and ZnO nucleation on the PAO template without metal catalysts. Field-emission scanning electron microscope images revealed that the nanorods have uniform length distributions and hexagon end planes, and the intense c-axis growth was also confirmed by X-ray diffraction. Strong ultraviolet emission at 380 nm and weak green band emission at 520 nm at room-temperature photoluminescence clearly indicated the high quality of the ZnO nanorods. A growth mechanism is proposed that the multipore surface of the PAO template plays a critical role in the nucleation of ZnO in the initial stage of growth, and nanorods grow from the nuclei due to intense ZnO c-axis orientation growth.